Cellular respiration

Cellular respiration is what cells do to break up sugars into a form that the cell can use as energy. This happens in all forms of life. Cellular respiration takes in food and uses it to create ATP, a chemical which the cell uses for energy. Regular cellular respiration is aerobic (requires oxygen), but some simple organisms can only do anaerobic cellular respiration.

Biology textbooks often state that 38 ATP molecules can be made per oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle, and about 34 from the electron transport chain).[1] However, this maximum yield is never quite reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into the mitochondrial matrix. Present estimates are 29 to 30 ATP per glucose.[1]

Aerobic metabolism is about (see sentence above) 15 times more efficient than anaerobic metabolism. Anaerobic metabolism yields 2 mol ATP per 1 mol glucose. They share the initial pathway of glycolysis but aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.

Pyruvate from glycolysis is actively pumped into mitochondria. One carbon dioxide molecule and one hydrogen molecule are removed from the pyruvate (called oxidative decarboxylation) to produce an acetyl group, which joins to an enzyme called CoA to form acetyl CoA. This is essential for the Krebs cycle.

Acetyl CoA joins with a chemical to form a compound with six carbon atoms. This is the first step in the ever-repeating Krebs cycle. Because two acetyl-CoA molecules are produced from each glucose molecule, two cycles are required per glucose molecule. Therefore, at the end of two cycles, the products are: two ATP, six NADH, two QH2, and four CO2. The ATP is a molecule which carries energy in chemical form to be used in other cell processes.

This is where most of the ATP is made. All of the hydrogen molecules which have been removed in the steps before (Krebs cycle, Link reaction) are pumped inside the mitochondria using energy that electrons release. Eventually, the electrons powering the pumping of hydrogen into the mitochondria mix with some hydrogen and oxygen to form water and the hydrogen molecules stop being pumped.

Eventually, the hydrogen flows back into the cytoplasm of the mitochondria through protein channels. As the hydrogen flows, ATP is made from ADP and phosphate ions.[1]